Oil is coated on rolled metal sheets, e.g. aluminum for can material or other sheet metals, to provide auxiliary lubrication during the press forming process. This operation is called reoiling, and has conventionally been carried out by continuously spraying oil over the entire surface of the running sheet. Non-uniformity of oil films formed on sheets can cause defects in quality, such as black streaks, so the measurement of quantity or thickness of the oil film has become a very important factor in the control of the manufacturing process.
In the past, measurement of oil film thickness was carried out using the following methods: measuring the weight difference of a test piece before and after the removal of oil film; the FT - IR method whereby the amount of infrared ray absorbed by the oil film surface is measured; and the atmospheric-type ultraviolet photoelectron detection method whereby ultraviolet rays are applied to a test piece and photoelectron emissions are measured.
Measuring of the thickness of paint film coated on rolled metal sheet was carried out using the following methods: the infrared ray absorption method whereby the characteristic of absorbing a definite range of infrared ray wave length of coated paint is utilized; and the .beta. ray diffusion method whereby .beta. ray is applied to the paint film and the amount of .beta. ray diffused or scattered by the paint film is measured because the amount of .beta. ray diffusion is closely connected with the thickness of paint film.
The above-mentioned off-line oil film measuring methods, however, do not permit assessment of the entire length and width of the sheet, because only locally sampled test pieces are examined. In addition, the weight or FT - IR methods are burdened by the considerable time and effort needed for the preparation of measurements; and the atmospheric-type photoelectron detection method by ultraviolet ray is limited by the thinness of the oil film when measuring.
The problem of the above paint film thickness measuring methods is the inferiority in measuring accuracy. And another problem is that the measuring place on the paint film is very limited spot. It also results in inaccurate measurement.